If u(x) = f (x) + it(x) is a complex-valued function of a real variable x and the real and imaginary parts f (x) and g(x) are differentiable functions of x, then the derivative of u is defined to be u'(x) = f'(x) + ig'(x). Use this together with Equation 7 to prove that if F(x) = e^rx, then F'(x) = re^rx when r = a + bi is a complex number.